As disclosed in JP2002-82549A, for example, there has been used a fixing device of electromagnetic induction heating type in an image forming apparatus such as a copying machine and printer, with the purpose of saving energy by reducing a starting up time of apparatus.
In the above JP2002-82549A, the fixing device of electromagnetic induction heating type comprises a supporting roller (heat generation roller), a fixing assist roller (fixing roller), a fixing belt stretched to the supporting roller and the fixing assist roller, an induction heating portion which faces the supporting roller via the fixing belt and a pressure roller which faces the fixing assist roller via the fixing belt. The induction heating portion comprises a coil portion (exciting coil) which is disposed to extend in the width direction (direction orthogonal to a transferring direction of recording medium) and a core which faces the coil portion.
The fixing belt is heated in a position facing the induction heating portion. The heated fixing belt heats a toner image on the recording medium which is transferred between the fixing assist roller and the pressure roller, so as to fix the toner image on the recording medium. More particularly, if high-frequency alternating current is applied to the coil portion, a magnetic field is formed around the coil portion; thereby, eddy current is generated adjacent to the surface of supporting roller. If the eddy current is generated to the supporting roller, Joule heat is generated by the electric resistance of the supporting roller itself. Accordingly, the fixing belt winded around the supporting roller is heated by Joule heat.
There has been known that such a fixing device of electromagnetic induction heating type can rise a surface temperature (fixing temperature) of the fixing belt to a desired temperature in a short starting up time with low energy
On the other hand, there has been disclosed in JP2000-214703A a fixing device using an electromagnetic induction heating type which forms a core portion to sandwich a fixing belt. More particularly, the core portion of induction heating portion is disposed to face the outer circumference plane and the inner circumference plane of the fixing belt. This technique is aimed at improving the exothermic effect of the fixing belt.
There has been also disclosed in JP2000-162912A a fixing device using an electromagnetic induction heating type, which adjusts Curie point of a core portion (magnetic core) of induction heating portion in the width direction. More particular, the fixing device is formed such that Curie point of the core portion in the both end portions in the width direction becomes smaller than Curie point of the central portion in the width direction. This technique is aimed at controlling temperature up which occurs in the both end portions in the width direction of the fixing belt when a small size recording medium passes.
With respect to the above fixing devices, the present inventors has proposed a fixing device that a coil portion is arranged to sandwich front-back both planes of a heat generation member when using a member having Curie point (member capable of controlling a self temperature) as the heat generation member in order to prevent the excessive temperature up of heat generation member, because such arrangement increases the self temperature control performance of heat generation member, compared to when the coil portion which generates magnetic flux is arranged to face the main plane of heat generation member (reference to Japanese Patent Application No. 2005-140195).
As a device structure, it is necessary for the above fixing device to sandwich the front-back both planes inner and outer circumference planes) of heat generation member by means of a loop-shaped coil portion. The performance can be fully shown by this arrangement. When the fixing device using the electromagnetic induction heating type is utilized, the heat generation body is used as the supporting roller and the fixing belt is winded therearound, so it is necessary to support the supporting roller in a rotatable manner. More particularly, it is necessary to dispose a continued loop-shaped coil which passes the front-back both planes of the rotatably supported supporting roller having the fixing belt.
A litz wire is generally utilized as the loop-shaped coil. In order to ensure the fixing performance, it is necessary for the number of winding of litz wire to secure five times or more at minimum. In order to obtain this structure, at first, in a state that the supporting roller inserted into the inner circumference plane side of the fixing belt and a shaft bearing for rotating the supporting roller are disposed, it is necessary to repeat five times or more a step of passing the litz wire from the inner circumference plane to the outer circumference plane of the supporting roller and again passing the wire from the inner circumference plane. Moreover, in order to maintain the heat generation performance, it is necessary to arrange the litz wire in a correct position in series.
Explaining these steps as a practical operation, the litz wire having ø3 mm in an outer diameter is repeatedly inserted five times into the inner circumference plane side of the supporting roller having ø28.5 mm in an inner diameter to be retained in a predetermined position. Such an operation requires a lot of time. Moreover, it is difficult to avoid the generation of scratch to the already disposed litz wire caused by the insertion and contact of the litz wire. That is, the mass productivity is extremely low.
Since the assembled loop-shaped coil, fixing belt, supporting roller and shaft bearing are integrated, it has a poor handling ability. If a defect of component is found out which requires changing the component, it is almost impossible to change the component.
In addition, since the litz wire comprises a wire which bundles about 100 wires, each having about ø0.1 mm, the loopback of litz wire is required in the fixing device so as to obtain the number of winding of the coil. However, the litz wire can not be bent at a right angle because the litz wire is strong As a result, unnecessary spaces are required for placing the litz wire outside of the both end portions of supporting roller.